The present invention relates to snubber circuits. In particular, the present invention relates to a non-dissipative snubber circuit for a switched mode power supply.
Snubber circuits are used in various power circuits to attenuate the magnitude of parasitic voltage spikes that develop in these supplies. FIG. 1 is a schematic of a conventional switched mode power supply having a two transistor forward topology. Switched mode power supplies are utilized to convert alternating voltage (a.c.) to direct voltage (d.c.) for various types of loads such as computers and telecommunication equipment. A transformer 10 is used to provide this isolation. On the primary side, an alternating voltage source 12 is converted to a direct voltage signal by rectifying bridge 14 and input capacitor 16. Transistors 18 and 20 are cycled on and off with a variable duty cycle which converts the dc voltage provided by the input capacitor to an ac voltage which the transformer 10 can use. The secondary winding 100 of the transformer 10 provides both isolation from the primary and the ability to select the voltage level on the secondary by the turns ratio of the secondary winding 100. The output voltage V.sub.o is regulated by changing the duty cycle of the transistors 18 and 20. A control circuit (not shown) senses the output voltage and controls the on time of transistors 18 and 20 to keep output voltage V.sub.o at a fixed level independent of any input line and output load variation as is well known to those of ordinary skill in the art.
The components of the secondary circuit will now be described followed by a description of the operation of the circuit. The secondary of this switched mode power supply includes a secondary winding 100, a first diode 102, a second diode 104, an output inductor 106 and an output capacitor 108. The first diode 102 is preferably referred to as a rectifying diode and the second diode 104 is preferably referred to as a freewheeling diode. The secondary winding 100 preferably has one end connected to ground and the other end connected to the anode of the rectifying diode 102. The freewheeling diode 104 is connected in parallel with the secondary winding 100 and rectifying diode 102. The output inductor 106 and output capacitor 108 are connected in series across the freewheeling diode 104 as shown. The output of the power supply is taken across the output capacitor 108.
The operation of the secondary of the power circuit will now be described. The primary side of the transformer transmits a square wave pulse train of on and off pulses to the secondary side of the transformer. The secondary winding 100 receives the pulse train of on and off pulses, and, during an on pulse, a voltage V is developed across the secondary winding 100. When an on pulse is received by the secondary winding 100, rectifying diode 102 is forward biased and freewheeling diode 104 is reversed biased. Current flows from the secondary winding 100 through rectifying diode 102 to the output inductor 106 and output capacitor 108 to charge inductor 106 and capacitor 108 and deliver power to the load (not shown) which is connected across capacitor 108. When an off pulse occurs, the voltage on the secondary winding 100 collapses (i.e. goes to zero), rectifying diode 102 becomes reversed biased and freewheeling diode 104 becomes forward biased. A current path is thus established between output inductor 106 and capacitor 108 and freewheeling diode 104. Thus during the off pulse cycle, energy stored in the inductor 106 and capacitor 108 is supplied to the load.
A problem arises, however, each time the secondary winding 100 of the power transformer receives an on pulse. In particular, when the next on pulse is received by the secondary winding 100, rectifying diode 102 is forward biased and freewheeling diode 104 is reversed biased. When freewheeling diode 104 is reversed biased, it tries to turn off, however, the charge stored in the P-N junction of the diode 104 must be swept out before the diode 104 can turn off. Without a snubber circuit in place, the freewheeling diode 104 can generate a large, fast voltage spike, often referred to as a parasitic voltage spike, as it tries to turn off. If the voltage spike is too large, the diode will fail due to an over-voltage condition. If the voltage spike is too fast, electrical noise is generated which can cause excess electromagnetic interference (EMI).
FIG. 2 is a schematic of the secondary of a switched mode power supply including a snubber circuit 110A according to the prior art. The snubber circuit 110A includes a capacitor 112 and a resistor 114 connected in series across the freewheeling diode 104. During the on pulse cycle, the capacitor 112 of the snubber circuit 110A also charges up to a voltage about equal to the voltage across secondary winding 100. During the off pulse cycle the energy stored in the capacitor 112 of the snubber circuit 110A dissipates through resistor 114. The snubber circuit 110A helps control the size of the spike caused by the freewheeling diode 104 turning off by providing a path for the charge developed across the P-N junction of the freewheeling diode 104 to flow into. Thus the capacitor 112 stores the energy of the spike generated by the freewheeling diode. During the next off pulse this energy is also dissipated in resistor 114.
FIG. 2A illustrates a typical waveform seen across freewheeling diode 104 as it tries to turn off. Depending upon the size of the diode and the environment in which the power circuit is placed, the amount of energy generated by the turn-off of the freewheeling diode 104 can be significant. The duration of the spike is very small compared to the duration of the entire on pulse. The total power lost in this circuit consists of two components. The first is the power generated by the charging and discharging of snubber capacitor 112. The first component may be defined by the equation 1/2 CV.sup.2 f, where V is the secondary voltage generated by secondary winding 100, f is the frequency of the AC signal transmitted by the primary winding and C is the capacitance of capacitor 112. The second component is the power generated by the voltage spike which is caused by the freewheeling diode turning off.
It is thus desirable to utilize the excess energy generated by the freewheeling diode 104 turning off instead of dissipating it in a lossy component such as resistor 114. In particular, it is preferable to design a snubber circuit that allows the excess energy developed from the freewheeling diode turning off to be directed to the output (i.e., load) or back to the input.
Accordingly, it is an object of the present invention to provide a snubber circuit with favorable characteristics.